238 



On the Silica Strata of the Loiver Chalk. 



difficulty and expense of making alkaline silicates for agricultural 

 use arises from the necessity oi fusion of the materials at a high 

 temperature. The process is essentially a glass-making process, 

 but, in addition to the ordinary costliness of the operation, the 

 excess of alkali necessary for making soluble glass is exceedingly 

 destructive to the pots and kilns. 



The silica rock of Farnham may be made to combine with 

 soda or potash in two or more ways without fusion — either,* first, 

 by boiling it with caustic potash or soda for a short time, when 

 the silica dissolves, and the solution can be separated and evapo- 

 rated to any required strength, or to dryness; or, secondly, by 

 heating the rock with a proper proportion of crude carbonate of 

 soda at a gentle temperature in a reverberatory furnace. No fusion 

 occurs, or is necessary, since, from the nature of the silica, its 

 perfect combination with the soda takes place at a very low 

 temperature. 



In this latter process (which would be by far the most econo- 

 mical) the alkaline silicate is at once obtained in the dry state. 

 It would of course not be pure, being mixed with the clay of the 

 silica rock ; still, if it became an article of commerce, it would 

 be sold, like other such impure compounds, at its fair value in 

 real silicate. We believe, then, that if silicate of potash and 

 soda are wanted for agriculture, we have at length an easy and 

 economical method of obtaining them. 



But in either of the above salts it is the silica principally, and 

 not the alkali, that is needed, and for which the silicate would 

 be used in agriculture ; and we shall have made a great advance 

 if we can substitute for the costly alkalies, potash and soda, a 

 cheaper substance to unite with the silica, such as lime. 



If there is a difficulty in making soda and silica unite, and a 

 necessity for a very high temperature, much more is this the case 

 with lime ; and, further, silicate of lime, when so formed, would 

 not be of any use to agriculture, because it is insoluble. We 

 have, however, quite overcome this difficulty by the discovery of 

 the silica rocks of Farnham. 



If to a solution of lime — that is to say, to ordinary lime-water 

 — you add a quantity of pure soluble silica, and agitate it for a 

 short time, you will find (by proper tests j) that the greater part 

 of the lime is removed, and that what still exists in the solution 

 is not in the condition of caustic lime, but of silicate. The 

 action is much more rapid if heat is applied. 



If, instead of using lime-water, we employ slaked lime with 



* It is easy to make the alkali take up in this way two equivalents of silica, or, 

 in the case of soda, three times its weight. 



f Nitrate of silver gives a brown precipitate with caustic lime, but when all the 

 lime is converted into silicate the pi'ecipitate is pale yellow or white. 



